Monoflanges combine the event as high as three valves in an especially compact body, because of a precise network of internal passages and valve chambers. But what really happens in the monoflange valve, once installed?
In a chemical process a high response speed is necessary for most control applications. Among the variables that affect the response time may be the volume and the distance between process and instruments. If the medium to be measured is gas, and the process will fluctuate strongly at times or if the control is crucial, mounting the instrument near the process may be the solution.
Vibrations may also be critical, for example, if impulse lines are connected to a vessel. The longer the hook-up, the wider may be the amplitude of the vibration causing possible failures of the nozzle. A monoflange includes one, several needle valves in the compact, flange-shaped body, allowing a substantial reduction in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
According to the requirements of the plant it is installed in, the monoflange can incorporate one, several valves. In a monoflange with two valves (block & bleed), one valve (with a blue cap) isolates the procedure and another (with a red cap) regulates the venting of the medium trapped in the instrument. This is mostly found in applications which are relatively uncritical (e.g. low pressure) or where a first shut-off valve is provided just before the monoflange.
The safest configuration, and the main one we advise for aggressive media or critical operating conditions, may be the three-valve monoflange or the so-called double block & bleed (DBB), which features two shut-off valves in series and something valve for venting.
Monoflange functionality
The monoflange bodies are drilled internally with holes which connect the annular valve chambers.
The following picture illustrates the procedure within a DBB monoflange:
The flow enters the monoflange from the pipeline and stops below the initial shut-off valve [1];
When the first shut-off valve [1] opens, the flow proceeds towards the next shut-off valve [2] ; once the valve [2] is open, the instrument is thus linked to the process line;
When the first shut-off valve [1] is closed, the medium trapped between valve and instrument could be discharged via the vent valve [3] through the vent outlet. Both shut-off valves [1, 2] come in an angled position, that allows the flow to feed them.
The two shut-off valves allow a better isolation from the process: In case the initial shut-off valve does not isolate the medium properly, the second one will become a safety means against accidental leaks. In some cases, customer specifications don’t allow the medium to be in touch with the instrument when it’s not measuring. Because of this the medium shall be discharged using the vent line. In other cases ? because of the vent line ? instruments can be easily calibrated without dismounting them from the line.
Note
Humor on our valves can be found on the WIKA website or in the video What is a monoflange? When you have any questions, your contact will gladly help you.